1. Field of the Invention
The present invention relates to an apparatus for detecting the remaining charge in a battery, and in particular, to such a device for a vehicle.
This application is based on Japanese Patent Application No. Hei 11-17680, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In recent years, electric vehicles and hybrid vehicles have been developed in order to reduce the discharge of carbon dioxide, in consideration of the global warming effect. These vehicles have a motor which does not discharge any gas, and a battery for supplying electric energy.
The electric vehicle is continuously driven by the motor. The hybrid vehicle utilizes the motor to assist the engine, and is often driven only by the motor.
Accurately detecting the remaining charge in the battery that supplies energy to the vehicle is important for determining whether to start charging or not, and to detect the remaining available time and the life of the battery.
A conventional method detects the remaining charge in the battery by time-integrating a charging current to the battery and a discharging current from the battery to the motor. The remaining charge basically corresponds to the total of charge stored in the battery. The total of the supplied and released electrical charge can be calculated by the time-integration of the electric current value (the charging current is positive, and the discharging current is negative). The details of this method can be understood from Japanese Patent Application, First Publication No. Hei 63-208773.
It is well known that, during the end stage of the charging (when the remaining charge is sufficient) or during the end stage of the discharging (when the remaining charge is insufficient), the rate of change in the output voltage from the battery with respect to the remaining charge varies largely. The reason for this is that the inner resistance of the battery varies depending on the remaining charge.
One of the background techniques, which takes into account these characteristics, corrects the remaining charge according to the voltage output from the battery when the ratio of change in the output voltage with respect to the remaining charge varies significantly (at the end of the charging or discharging).
During the intermediate period between the end stage of the charging and the end stage of the discharging, the electric current output from the battery is time-integrated so that the remaining charge can be obtained. As the rate of change in the output voltage increases, inflection points appear when entering the end stage of the charging or discharging. Then, based on the output voltage, the remaining charge obtained by the time-integration of the electric current is corrected, to thereby obtain the accurate remaining charge. This technique is described in Japanese Patent Application, First Publication Nos. Hei 6-342044, and Hei 5-87896.
FIG. 9 is a diagram showing the change in the voltage output from the battery. In this figure, the horizontal axis represents the remaining charge, while the vertical axis represents the voltage output from the battery.
As shown in FIG. 9, when the remaining charge is between the end stage of the discharging (the remaining charge is below 20%) and the end stage of the charging (the remaining charge is 80%), as the remaining charge decreases, the output voltage varies at a relatively small amount of the rate of change, which is indicated by the curve denoted by reference character R1. Because the hybrid vehicle repeats the charging and discharging of the battery, the actual voltage varies as shown in the line denoted by reference character R2. The small amount of the rate of change means that, because the rate of change slightly varies, its average is small.
When the remaining charge enters the end stage of the charging (the remaining charge becomes above 80%), the rate of change in the output voltage increases. When entering the end stage of discharging (the remaining charge falls below 20%), the rate of change similarly increases. Thus, the voltage output from the battery varies depending on the remaining charge, and especially varies significantly during the end stages of the charging and discharging.
In general, the conventional electric current detector necessarily incurs an error. When the electric current is momentarily measured, the error may be permissible. However, in the time-integration of the electric current, the errors are also integrated so that the discrepancy increases as time passes. This discrepancy is not permissible in the time integration because it reduces the detection accuracy.
In the above-mentioned technique which improves the accuracy in the detection of the remaining charge, the map for correcting the battery's remaining charge is obtained based on the measurement in a steady state. Therefore, in the transient state (in which the charging and discharging of the battery are repeated so that the electric current varies), the measured values differs from those in the steady state, and the accuracy in the detection of the remaining charge is reduced due to the delay in response from the battery. That is, even when the electric current output from the battery and the regenerative electric current vary, the voltage from the battery cannot immediately become stable (cannot be immediately the value in the steady state), and attaining a stable value is delayed by a predetermined time. Thus, when detecting the remaining charge based on the voltage and electric current values measured momentarily, an error occurs in the detection.
In particular, the batteries used in the electric vehicle and the hybrid vehicle differ from those in electronic devices in that the charging and discharging of the battery are frequently repeated, thereby reducing the accuracy of the detection of the remaining charge.